1. Field of the Invention
The present invention relates to a scanning-type instrument which utilizes a charged particle beam and images a specimen by focusing the beam onto the specimen with an objective lens, scanning the beam across the specimen, detecting a signal arising from the specimen, such as secondary electrons, and displaying an image of the specimen based on the detected signal. The invention also relates to a method of controlling this instrument.
2. Description of Related Art
A scanning electron microscope, that is, a kind of scanning-type instrument utilizing a charged-particle beam, is designed to sharply focus a primary electron beam onto a specimen and to scan the beam within a given range on the specimen. The electron beam is directed at the specimen, resulting in secondary electrons. These secondary electrons are then detected. The resulting signal is sent to a CRT synchronized with scanning of the primary electron beam. In this way, a scanning image of the specimen is displayed.
The fundamental structure of such a scanning electron microscope is shown in FIG. 3. This microscope has an objective lens 101, an electron gun 102 for producing a primary electron beam EB and accelerating it, condenser lenses 103, and other components are mounted above the objective lens 101. The primary electron beam EB is sharply focused by the condenser lenses 103 and objective lens 101 and directed at a specimen 104. Scan coils 105 are disposed above the objective lens 101 to scan the primary electron beam EB in two dimensions across the specimen 104.
As the electron beam EB is directed at the specimen 104, secondary electrons are produced. These secondary electrons are collected by a mesh collector (not shown) to which a voltage of about 100 V is applied. The collected secondary electrons are guided to a secondary electron detector 106, which is composed of a corona ring 107, a scintillator 108, and a photomultiplier tube 109. A high voltage of the order of 10 kV is applied to the corona ring 107 from a corona ring voltage source (not shown). The secondary electrons are accelerated and made to strike the scintillator 108. The photomultiplier tube 109 converts the light from the scintillator 108 into an electrical signal.
The secondary electron signal detected by the secondary electron detector 106 is supplied as a brightness-modulating signal via an amplifier and known signal-processing circuitry (none of which are shown) for adjusting the brightness and contrast to a CRT (not shown) synchronized with the scanning of the primary electron beam. As a result, a scanning secondary electron image of a certain two-dimensional area on the specimen is displayed on the viewing screen of the CRT.
In this scanning electron microscope, the geometry of the objective lens 101 is an important factor in determining the instrumental resolution. Therefore, in order to improve the resolution, the aberration coefficient of the objective lens 101 must be reduced. Accordingly, the aberration coefficient is suppressed to within 3 mm by adopting an xe2x80x9cin-lensxe2x80x9d objective lens in which the magnetic field on the specimen is intensified. Alternatively, a xe2x80x9csemi-in-lensxe2x80x9d objective lens (strong magnetic-field objective lens) may be adopted.
With the in-lens objective lens, the specimen is placed within the magnetic field set up by the objective lens. With the semi-in-lens objective lens, a single magnetic lens field is produced below the bottom surfaces of the inner and outer polepieces. The specimen is placed within this magnetic lens field.
To reduce the effects of objective lens aberration, it is customary to decelerate the electron beam immediately before the specimen. For this purpose, the primary electron beam having increased energies is passed through the objective lens. This method is known as a retarding field method. The resolution can be enhanced further at low accelerating voltages. In one retarding field method, a negative voltage is applied to the specimen as disclosed, for example, in Japanese Patent Laid-Open No. 10-334844/1998.
Where a retarding field method is adopted in which a negative voltage is applied to a specimen, a voltage is applied to the specimen in the SEM imaging mode. In this case, the focus of the objective lens varies. Accordingly, in the case of a magnetic objective lens, it is necessary to vary the conditions under which the objective lens is focused by varying the objective lens current.
If the conditions under which the objective lens is focused are varied, some parameters (e.g., magnification, image rotational angle, and angular aperture control lens current) vary concomitantly. In the existing scanning electron microscope, if a voltage is applied to the specimen, these parameters and conditions are maintained. Therefore, at this time, other parameters and conditions varying concomitantly with the application of the voltage to the specimen need to be adjusted manually. The required operations are quite cumbersome to perform.
It is an object of the present invention to provide a scanning-type instrument which utilizes a charged-particle beam and automates adjustments, even if a voltage is applied to a specimen, to thereby provide excellent operational controllability. It is another object of the present invention to provide a method of controlling this instrument.
A scanning-type instrument which utilizes a charged-particle beam and built in accordance with the present invention comprises: an objective lens for focusing the charged-particle beam onto a specimen; deflection means for scanning the beam within a desired area on the specimen; and detection means for detecting a signal produced as the specimen is irradiated with the charged-particle beam. The instrument obtains an image of the specimen based on the signal detected by the detection means. The instrument is characterized in that it further includes a voltage application circuit for applying a voltage to the specimen and first control circuit for controlling the strength of the objective lens according to the voltage applied to the specimen. Defocus of the beam caused by the application of the voltage to the specimen is corrected under control of the first control circuit.
In a method of controlling a scanning-type instrument utilizing a charged-particle beam in accordance with the present invention, the scanning-type instrument has an objective lens for focusing the charged-particle beam onto a specimen; deflection means for scanning the beam within a desired area on the specimen; and detection means for detecting a signal produced as the specimen is irradiated with the charged-particle beam. The instrument obtains an image of the specimen based on the signal detected by the detection means. The method consists of applying a voltage to the specimen and controlling the strength of the objective lens according to the voltage applied to the specimen to thereby correct defocus of the beam caused by the application of the voltage to the specimen.
Other objects and features of the invention will appear in the course of the description thereof, which follows.